The invention relates to a color television display tube comprising, in an evacuated envelope, an electron gun system having three electron guns arranged in line to generate three converging electron beams. The electron guns each comprise, in the direction of propagation of the electron beams, a triode part, an elongate first lens electrode and a second lens electrode. The first lens electrodes are secured to insulating assembly rods extending substantially in the axial direction of the gun system by means of first and second axially-spaced metal suspension braces, respectively.
An electron gun system for such a color television display tube is described in German Offenlegungsschrift No. 2526210.
It has been found that in color television display tubes the convergence of the electron beams changes in the period between switching on the tube and the instant at which it reaches its operating temperature. This convergence drift is particularly a result of thermal expansion of the electrodes and the metal suspension braces connected thereto, so that a variation in the position of the electrodes of the two outermost guns with respect to that of the central gun may occur. Furthermore, in the case of a thermal expansion of a suspension brace, forces may be exerted on the assembly rods connected thereto. These forces are transferred by the assembly rods to suspension braces connected to other electrodes, so that the positions of the other electrodes vary.
U.S. Pat. No. 3,974,416 proposes to reduce the so-called thermal convergence drift by manufacturing at least the suspension braces of the control electrodes of the gun system from a metal having a low coefficient of linear expansion. This solution is satisfactory for gun systems in which the first lens electrode (referred to as focusing electrode 3 in FIG. 1 of the U.S. Pat. No. 3,974,416), has such a small axial length that it can be secured to the assembly rods by means of only one suspension brace. However, if the axial length of the first lens electrode is such that this electrode, in order to obtain a sufficient mechanical stability, has to be secured by means of two suspension braces situated at an axial distance from each other, then it is found nevertheless that an impermissible degree of thermal convergence drift occurs.